It is well known in the electrical field the use of protection devices, such as current switches, for example, circuit breakers or switch-disconnectors, which can be designed to switch an electrical system in which they can be installed for example, to protect the system from fault events, such as overloads and short circuits or for connecting and disconnecting a load.
Common electro-mechanical switching devices can include a couple of separable contacts to make, break and conduct current; in the breaking operation, a driving mechanism triggers the moving contacts to move from a first closed position in which they can be coupled to the corresponding fixed contacts, to a second open position in which they can be separated therefrom.
Usually, at the time the contacts start to physically separate from each other, the current continues to flow through the opened gap by heating up the insulating gas which surrounds the contacts themselves until the gas is ionized and becomes conductive, e.g., the so-called plasma state is reached; in this way, an electric arc is ignited between the contacts, which arc has to be extinguished as quickly as possible in order to definitely break the flow of current. For example, in direct current (“DC”) applications, the interruption time can be quite high, and electric arcs can consequently last for a rather long time.
Such long arcing times can result in severe wear of the contacts, thus reducing significantly the electrical endurance, e.g., the number of switching operations that a mechanical current switch can perform.
For example, in order to quickly extinguish the arc and minimize such problems, the flowing current can be decreased and with it the heating power below a certain threshold where the heating is not sufficient to sustain the arc; the plasma cools down and loses its conductivity.
In a low voltage DC circuit, the current is reduced by building up a countering voltage exceeding the applied system voltage. The built-up voltage, exceeding the system voltage, should be maintained until the current is switched off; this voltage is usually produced by splitting up the arc in many short segments using a series of splitter plates.
To this end, for standard LV circuit breaker geometries, the arc has to be moved from the ignition area, where the contacts open, to the arc chamber where the splitting plates can be positioned; this can be done by exploiting a magnetic field generating a Lorentz force on the arc column.
This magnetic field can be generated by the same current flowing through the switching device. However, while having a capability to extinguish electric arcs with very high short circuit currents, known mechanical current switches can struggle to build up voltages above a certain value, for example, 600-1000V, and can have difficulty to extinguish electric arcs when switching operations can be carried out at low currents, for example, a few tens of Amperes.
In these cases it is therefore possible that at low currents an electric arc continues to burn on the contacts without being moved away from the contacts towards the arc splitting plates: as a consequence, the arc voltage built up is low and current is neither limited nor interrupted.
In some circuit breakers, an additional permanent magnet is usually provided for strengthening the magnetic field which acts on the arc column to move it towards the arc splitting plates. However, in this case, in addition to issues related to cost, position and space availability for this additional component, the circuit breaker is only able to interrupt currents with a given polarity defined by the placement of the permanent magnet. If the current flows in the opposite direction the arc is kept at the contacts which can be worn by the arc continuously burning on them.
It other known implementations hybrid current switching devices can be used in which a known or main mechanical circuit breaker is connected in parallel to a semiconductor-based current switching device.
These hybrid solutions can be aimed at having ideally arc-less switching operations or at least the extinguishing of electric arcs as fast as possible.
To this end, when the contacts of the mechanical breaker have to be opened, the flow of current is commuted towards the semiconductor device. In known implementations, the semiconductor can be driven into its conductive state even before the contacts of the mechanical breaker can be actuated; in other ones, the semiconductor is driven into its conductive state immediately after the contacts of the mechanical breaker can be actuated in order to remove the arc from the mechanical contacts as early as possible.
Although such hybrid solutions perform quite well, one of their shortcomings is that the semiconductor device, when driven in the conductive state, is always exposed to and has to face the flowing current which can reach very high levels. As a result, there is a high risk of possible damages and in any case, because in many operative conditions currents involved can be rather high, protections schemes and/or rather expensive components can be adopted or used.